Owing to their design, flanges always employ a “seal” made of more or less rigid material capable of providing a hermetic seal locally. This part represents a weak point in the flange and becomes the main source of the leak. This defect is accentuated when the flange is operated under pressure. For this reason, pipe leaks often occur in flanges.
Repairing leaking flanges while maintaining the pipe in operation remains very delicate. In the case of colourless gas, the operation's complexity stems from the difficulty in locating the leak which may occur in the upper part of the seal, or travel through the mounting bolts. In such conditions, the leak remains undetectable.
There are currently three categories of repair processes although their efficiency depends on the flange's damage and particularly on the operating conditions (temperature, pressure, type of fluid . . . ). Thus, in the conventional mechanical process, two half-clamps are used to enclose the flange. These clamps, are often made of steel, heavy and bulky, create mechanical stresses and weaken the flange. This technology employs either elastomeric seals to be compressed when clamped around the flange, or an overmoulding resin injected into the void formed between the clamp and the flange. The repair thus condemns the flange as the resin is often a powerful adhesive that is very difficult to remove.
In the majority of cases, the leaking flanges are often inaccessible, obstructed, damaged and even sometimes ovalised. As a result, the use of a sealing clamp is not always obvious as it is bulky and heavy and always causes premature ageing of the pipe and leads to the formation of cracks.
If resin overmoulding is used, the metal casing creates the same stresses as previously described in addition to the difficulties related to the injection operation. Furthermore, it is impossible to inject any resin whatsoever while the flange is leaking as major adhesion problems appear. In addition, pressurized injection is not possible as the operator may accidentally force resin into the pipe.
A second process, used particularly in Great Britain, consists in injecting the resin. The flange is perforated and an adapted resin is injected into the leaking seal. The resin is supposed to travel around the flange, fill in any voids, polymerise despite the operating conditions and stop the leak. The use of this delicate and unpredictable technology remains rather limited in maintenance departments.
Finally, the “banding” process consists in applying a strip of glass fibre that has been pre-impregnated with resin. The application is difficult or even impossible under pressure. This “tourniquet” system around the leaking flange resembles more an emergency quick-fix than certified repair by professionals. This process presents major drawbacks. Particularly, it can be used only if the surface of the flange is prepared and cleaned to ensure proper adhesion of the composite (which cannot be guaranteed with an application in a leak environment) and, if installed correctly, prevents all access to the flange and prevents it from being dismantled during scheduled maintenance operations.